The present invention relates generally to systems and methods for magnetic resonance imaging (“MRI”) and, more particularly, the invention relates to systems and methods for image reconstruction in parallel MRI.
Contrast-enhanced magnetic resonance angiography (“CE-MRA”) is a method whereby magnetic resonance imaging (“MRI”) techniques are used to image blood vessels of the body after administering a contrast agent to the patient. Typically, a moderate amount of a gadolinium-based contrast agent is injected into a vein in the patient's arm. The contrast agent then makes its way into circulation through the patient's vasculature. The presence of the contrast agent in the blood causes the net relaxation time of the blood to be altered from its unenhanced value. MR acquisition methods can exploit this change in relaxation time, causing the enhanced blood within the vasculature to be significantly brighter compared to other structures within the imaging field-of-view (“FOV”).
There are technical challenges associated with performing CE-MRA. First, to obtain a three-dimensional image with adequate spatial resolution, it is necessary to have a sufficiently long acquisition time. Depending on the FOV and the desired spatial resolution, the time necessary to provide the degree of sampling required to achieve this spatial resolution can range from ten seconds to several minutes. Second, the initiation of the MRI data acquisition must be matched to the arrival of the contrast-enhanced blood within the vessels of interest, and this injection-to-arrival time is variable from patient to patient. Third, it is generally desirable to generate an angiogram in which there is negligible contrast enhancement within the companion venous system. These challenges have been addressed in various ways. For example, short repetition time (“TR”) gradient echo sequences allow rapid collection of MRI data. Synchronizing the acquisition to the contrast arrival can also be done using a test bolus or fluoroscopic triggering. Extension of the acquisition duration well into the venous phase, but with intrinsic suppression of venous signals, can be done using various centric phase encoding view orders.
Parallel imaging is a method whereby the redundancy in samples collected from multiple receiver coils is used to reduce the number of repetitions of the pulse sequence, and thus the acquisition time, that is necessary to generate an image with a given spatial resolution. Parallel imaging is generally implemented by a modification of the sampling of k-space along one or more phase encoding directions. These phase encoding directions commonly the ky direction for two-dimensional acquisitions, and both the ky and kz directions for three-dimensional acquisitions. Although parallel imaging can be implemented in non-Cartesian MR acquisitions, most applications to date have used Cartesian approaches with 2DFT or 3DFT sampling.
The degree of undersampling provided by a parallel acquisition is referred to as the acceleration, R. For a 3DFT acquisition, the undersampling can be applied separately along both the ky direction, providing an acceleration Ry, and along the kz direction, providing an acceleration Rz. Undersampling in two directions like this results in an overall acceleration of R=Ry×Rz. The reduction in acquisition time achievable with parallel imaging acquisitions has allowed time-resolved methods to be used with frame times in the 5-10 second range, and with spatial resolution superior to that of non-accelerated acquisitions.
Implementation of parallel acquisition requires extra data and extra mathematical processing beyond that of standard image reconstruction. The extra data includes images of the sensitivity profiles of the individual receiver coils over the object. For image-space-based approaches to parallel acquisition, such as SENSE, the coil sensitivity maps are generated from separate acquisitions, generally made before the SENSE-accelerated scan. For k-space-based approaches to parallel acquisition, such as GRAPPA, the additional data is acquired within the accelerated acquisition, increasing the overall number of points acquired and forcing the acceleration, R, to be reduced to some smaller value, Rnet. The key point of this discussion is that for both approaches to parallel imaging, there is overhead time required for the implementation of the parallel acquisition, primarily due to the need to acquire calibration data.
It would therefore be desirable to provide a method for parallel imaging in which calibration data could be acquired without a reduction in acceleration, R, and without additional constraints on data acquisition time.